Hundreds of patients at Hawke's Bay Hospital could have eaten the pre-packaged meat found to contain the potentially lethal listeria bug.

Meat products from a company where listeria was found have also been supplied to rest homes, though the investigating authority will not say which ones.

There have been four reported cases in the region since May. The bug has claimed the life of a woman in her 60s and contributed to the death of another in her 80s.

Listeria can be lethal to the elderly, the pregnant and those with decreased immunity.

The incubation period can be up to 70 days and local doctors and midwives have been advised by Hawke's Bay District Health Board that the illness may develop in patients over the next few weeks.

Investigations found the bug in pre-packaged ready-to-eat meats supplied to the hospital kitchen. A connection has not been made between the meats and the deaths, but results from DNA samples should confirm whether there is a link early next week.

A health board spokeswoman said there was no way of knowing how many patients had been exposed to the bug. The first case was notified on May 9, but the source of infection was still uncertain.

"People choose these meats off a menu. A record isn't kept of who ate what," she said. The meats were not served to the maternity ward.

The Primary Industries Ministry is investigating Napier company Bay Cuisine, where listeria was found in some products. The company is the sole supplier of pre-packaged meats to the hospital.

Ministry deputy director-general compliance and response Andrew Coleman could not say how long the investigation would take.

He would not speculate on a link between the company and the infections, but said "establishing such a link is part of our inquiry".

He said it appeared some of the products in question had been supplied to rest homes, but he would not say which ones.

Any person or organisation found to have been the source of listeria could face a fine of up to $500,000 under the Animal Products Act, if careless, reckless or intentional breaches were detected.

Bay Cuisine director Simon Wills yesterday said the recall of Mad Butcher salami and pepperoni, Ratanui hams and Ezy Carve ham pieces was well under way.

The health board had not informed Bay Cuisine which products had been found to be infected, so he could not say where else it had been supplied.

An 81-year-old Napier woman infected by listeria about two months ago died on July 9.

Her husband said he did not know how she picked up the infection. "It could have come from anywhere."